In the conventional process for producing halobutyl rubber, isobutylene and isoprene monomers are first polymerized in a polar halohydrocarbon medium, such as Methyl Chloride (MeCl) with an aluminum based initiating system, typically either aluminum trichloride (AlCl3) or ethyl aluminum dichloride (EtAlCl2). The butyl rubber does not appreciably dissolve in this polar medium, but is present as suspended particles and so this process is normally referred to as a slurry process. Residual monomers and polymerization medium are then steam stripped from the butyl rubber, before it is dissolved in a halogenation medium, typically a non-polar medium such as hexane (C6). The halogenation process ultimately produces the final chlorinated or brominated product. The conventional process therefore employs separate polymerization and halogenation steps employing two different media. The use of a polar medium for polymerization and a non-polar medium for halogenation necessitates intermediate stripping and dissolving steps and is inefficient from an energy point of view.
The step of separating the monomers and MeCl from the butyl polymer is conducted before halogenation in order to avoid the formation of highly toxic byproducts from the reaction of chlorine or bromine with residual monomers. The normal boiling points of the components used in the process are: MeCl, −24° C.; isobutylene, −7° C.; and, isoprene, 34° C. Any stripping process that removes the heavier of the residual monomers (isoprene) will also remove essentially all of the MeCl and isobutylene. The process of removing all of the un-reacted components from the rubber slurry requires significant amounts of energy. The greater molecular weight (and therefore higher boiling point) of the halogenated monomers also precludes the removal of these species following the halogenation process.
Solution processes for the polymerization of butyl rubber have been known for many years and are practiced commercially in Russia. An example of the solution process is described in Canadian patent 1,019,095, which discloses the use of iso-pentane (C5) as the preferred polymerization medium. The polymers produced using the above process are non-halogenated. Although halogenation could take place in the saturated C5 medium, the presence of residual monomers (isobutylene, iC4, and isoprene, iC5) would lead to formation of the afore-mentioned undesirable by-products during halogenation. The removal of the unreacted monomers is the challenge for such a process and has not been resolved yet. Although it would be desirable to remove the monomers by distillation, the boiling points of the C5 medium (isopentane, 28° C.) is lower than that of the heavier residual isoprene monomer (iC5, 34° C.), therefore this kind of separation is impossible. Even if normal pentane (boiling point 36° C.) were used as the medium, the difference in boiling points would be insufficient to allow effective removal of the isoprene using distillation techniques. As a result, the residual monomers and medium would all have to be stripped together from the butyl rubber, as in the slurry process, with the rubber being subsequently re-dissolved for halogenation. This is, in fact, more energy intensive than halogenation from the conventional slurry process. The use of isopentane (C5) as a common medium for producing halobutyl rubber is therefore not practical using the conventional solution process.
It is known in the art to use hexane (C6) as a polymerization medium in the solution process. However, the viscosity of a polymer solution is strongly dependent upon the viscosity of the medium used. Because the viscosity of a C6 medium is much higher than that of a C5 medium, for a given molecular weight and polymer solids level, the resulting viscosity of the polymer solution is also much higher. This limits polymer solids content to relatively low levels when C6 is used as a solvent, since otherwise the solution becomes too viscous for good heat transfer, pumping and handling. The overall economics of a process depend strongly on the level of polymer solids in the solution or suspension emerging from the polymerization reactor; higher solids levels mean higher conversion and improved economics. In order to make material having a sufficiently high molecular weight for commercial purposes, it is necessary in butyl polymerization to employ relatively low temperatures, often less than −80° C. These low temperatures exacerbate the problem of high solution viscosity and lead to even lower solids levels. In the solution process, it is therefore quite difficult to achieve an economic solids level (conversion) at the desired temperature (molecular weight) when using hexane as a solvent due to high viscosity.
In U.S. Pat. No. 5,021,509 a process is disclosed whereby product from the conventional slurry polymerization process is mixed with hexane to produce a crude rubber solution or cement. The hexane is added to the MeCl-rubber slurry after exiting the polymerization reactor in order to dissolve the rubber in hexane while still finely divided and suspended in the MeCl/monomer mixture. A distillation process is then used to remove MeCl and residual iC4 and iC5 monomers for recycle, leaving just the rubber in a hexane solution ready for halogenation. This so-called “solvent replacement” process still requires that all of the original media left with the rubber after the polymerization stage are removed. The energy requirement is essentially the same as in the conventional process. No common solvent is employed for both polymerization and halogenation.
There still remains a need for a process that uses a common solvent for both polymerization and halogenation, particularly a process that operates within an acceptable range of viscosities in order to allow high rubber solids levels at the desired molecular weight. The process must allow separation of the residual monomers from the solvent prior to halogenation in order to mitigate the formation of undesirable by-products.